1. Field of the Invention
The present invention generally relates to an ink droplet expelling apparatus which can be applied to a non-impact printing apparatus, and particularly relates to an improved ink droplet expelling apparatus in which printing can be performed at a high speed as well as stably when the ink droplet expelling apparatus is used in a drop on-demand ink jet recording apparatus.
2. Description of the Prior Art
Conventionally, an apparatus in which ink droplets are expelled in accordance with printing signals so as to perform desired printing on a recording medium, that is, a so-called drop on-demand ink jet expelling apparatus has been known and a basic arrangement thereof is disclosed, for example, in U.S. Pat. No. 3,946,398 filed by Kyser et al. Further, an ink droplet expelling head in which a backflow of ink into an ink nozzle is suppressed immediately after an ink droplet has been expelled is disclosed, for example, in Japanes patent application unexamined publication No. 52-109935 laid open Sept. 14, 1977, filed in Japanes Patent Office claiming convention priority right on the basis of West German Application No. P 2555738.7 filed Dec. 11, 1975.
Each of those apparatuses has a simple arrangement having a single ink chamber and a single electro mechanical transducer for applying a pressure to the single ink chamber, so that ink droplets are expelled in accordance with pulses applied to the single electro-mechanical transducer FIG. 1 shows a general example of the conventional ink droplet expelling head having such a simple arrangement as described above.
In the cross sectional view (a) of FIG. 1, an ink droplet expelling head 1 is arranged such that a glass plate 3 is stuck to a silicon plate 2 formed with a groove 2a so as to form an ink chamber 4 with its front end communicated with an expelling nozzle 5 and with its rear end communicated with an ink tank 9 through a 3oint 6 and a pipe 7, and that an electro-mechanical transducer 8 for bending the silicon plate 2 to decrease the volume of the ink chamber 4 is stuck to the outer surface of the silicon plate 2. As the electro-mechanical transducer 8, mainly used is a piezo-electric device which is adapted to be deformed to bend the silicon plate 2, as shown in the partial cross-sectional view (b) of FIG. 1, in response to a driving pulse voltage applied thereto from an electric circuit (not shown).
FIG. 2 shows a process of expelling of an ink droplet out of the expelling nozzle 5 when an electric signal (a driving pulse voltage) is applied to the piezo-electric device 8 as the time elapses through the views (a) to (e). The view (a) of FIG. 2 shows the state where no voltage is applied to the piezo-electric device 8. The view (b) of FIG. 2 shows the state where a voltage is applied to the piezo electric device 8 so as to cause the piezo-electric device 8 to begin to deform to increase the pressure in the ink chamber 4 so as to expel ink 10. The view (c) of FIG. 2 shows the step where the voltage applied to the piezo-electric device 8 is removed and the piezo-electric device 8 is restored to its original shape, so that the ink chamber 4 becomes to have a negative pressure so that almost all the part of the ink 10 separated from an expelled ink portion is sucked back into the ink chamber 4 and the expelled ink portion flies toward a recording medium (not shown) in the form of an ink droplet 10a. Thereafter ink is supplemented to the ink chamber 4 from the ink tank 9 through the pipe 7. At that time, as shown in the views (d) and (e) in FIG. 2, the ink droplet 10a gradually becomes substantially spherical due to a surface tension thereof, during its flying. Sometimes, there occurs such a phenomenon that very small size ink droplets (satellites) follow the ink droplet 10a, in the case where the voltage of the driving signal applied to the piezo-electric device 8 is made higher.
One of the conditions required for the performance of the ink droplet expelling head is that a single ink droplet having a predetermined size should be expelled in accordance with an electric signal at a high speed as much as possible. In order to increase the expelling speed of the ink droplet, in the conventional ink droplet expelling head 1 as shown in FIG. 1, there has been proposed only the method of increasing the voltage applied to the piezo-electric device 8 so as to increase the amount of deformation of the piezo-electric device 8, that is, the amount of deformation of the silicon plate 2. When the amount of deformation of the silicon plate 2 increases, however, a negative pressure in the ink chamber 4 increases when the silicon plate 2 is restored to its original shape after the applied voltage was removed, so that a backflow of ink into the ink chamber 4 in the state of the view (c) of FIG. 2 becomes large. Therefore, air bubbles may be mixed into the ink 10 through the expelling nozzle 5 to thereby deteriorate the stable ink expelling condition thereafter and at last it may becomes impossible to perform the ink expelling operation. Due to such a problem, there is a limitation in expelling speed of the ink droplet 10a, that is about 3-3.5 m/sec at highest.
A second one of the conditions required for the performance of the ink droplet expelling head is that in order to perform printing at a high speed as much as possible, the ink expelling time intervals between adjacent ink droplets should be shortened. That is, the frequency of the electric signal applied to the piezo-electric device 8 is made high as much as possible.
FIG. 3 shows a frequency versus voltage characteristic of the driving signal applied to the piezo-electric device 8 of the ink droplet expelling apparatus having the conventional arrangement as shown in FIG. 1. Here, a curve s designates the minimum driving voltage (a threshold value) for making it possible to expel ink out of the expelling nozzle 5, while a curve u designates the maximum driving voltage for normally expelling a single ink droplet. When the applied voltage is made higher than the maximum driving voltage, the amount of deformation of the piezo-electric device 8 becomes so large that there occurs such a disadvantage that air bubbles may enter the ink chamber, or alternatively a plurality of small ink droplets may be expelled. It is desirable for this characteristic that a distance between the curves s and u is large as much as possible and constant even in the case where a driving frequency f is made higher.
As seen from FIG. 3, in the conventional ink droplet expelling apparatus, there was a limitation in frequency for performing stable ink droplet expelling with a predetermined voltage and it was impossible to drive the conventional ink droplet expelling apparatus with frequencies above a predetermined frequency f.sub.0, resulting in limitation in printing speed. The limitation in frequency is caused by the transitional pressure fluctuation within the ink chamber 4 immediately after the ink droplet expelling. The pressure fluctuation in the ink chamber 4 is caused by free vibrations of the silicon plate 2 which continue for a time even after the voltage applied to the piezo-electric device 8 has been removed and by an acoustic effect due to a pressure wave propagated through the ink chamber 4.
FIG. 4 shows examples of such a pressure fluctuation as described above. Now, when the piezo-electric device 8 is driven by a driving pulse I (an electric signal), the pressure fluctuation in the ink chamber follows an attenuation curve as shown by a solid line a. Then, if the piezo-electric device 8 is driven by a driving pulse II with a frequency f.sub.1, the pressure fluctuation a due to a driving pulse I and the pressure fluctuation b due to the driving pulse II are composed of each other into the pressure fluctuation b', so that the pressure in the ink chamber becomes larger than a normal value (a or b). Further, if the frequency of the driving pulse is selected to be f.sub.2, the pressure fluctuation a due to the driving pulse I and the pressure fluctuation c due to a driving pulse II' are composed of each other into the pressure fluctuation c', so that the pressure fluctuation in the ink chamber becomes smaller than the normal value. The unstable characteristic as shown in FIG. 3 is caused by such operations as described above.
Further, such a technique that an ink chamber is divided into two sections which are separately provided with individual piezo-electric devices to which electric signals individually applied to cause the piezo-electric devices to expel ink droplets, is disclosed in Japanese patent application Unexamined Publication No. 56-146765 filed in Japanese Patent Office Apr. 16, 1980 by the same assignee as the present application and laid-open Nov. 14, 1981. In this arrangement, the voltage of the individual electric signal is lowered and the negative pressure in the ink chamber after the voltage has been removed is decreased, so that a backflow of ink is suppressed, and at the same time, a synergistic effect due to the driving performed by the two electric signals increases the ink expelling speed. In this method, however, no measures for suppressing the pressure fluctuation in the ink chamber after the removal of the applied voltage is taken into consideration and there is a limitation in increase of driving frequency.